In the oil and gas industry, geophysical prospecting techniques are commonly used to aid in the search for and evaluation of subterranean hydrocarbon deposits. Generally, a seismic energy source is used to generate a seismic signal that propagates into the earth and is at least partially reflected by subsurface seismic reflectors (i.e., interfaces between underground formations having different acoustic impedances). The reflections are recorded by seismic detectors located at or near the surface of the earth, in a body of water, or at known depths in boreholes, and the resulting seismic data may be processed to yield information relating to the location of the subsurface reflectors and the physical properties of the subsurface formations.
One type of geophysical prospecting utilizes an impulsive energy source, such as dynamite or a marine air gun, to generate the seismic signal. With an impulsive energy source, a large amount of energy is injected into the earth in a very short period of time. Accordingly, the resulting data generally have a relatively high signal-to-noise ratio, which facilitates subsequent data processing operations. On the other hand, use of an impulsive energy source can pose certain safety and environmental concerns.
Since the late 1950s and early 1960s, a new type of geophysical prospecting, generally known as “VIBROSEIS”® prospecting, has been used. Vibroseis prospecting employs a land or marine seismic vibrator as the energy source. In contrast to an impulsive energy source, a seismic vibrator imparts a signal into the earth having a much lower energy level, but for a considerably longer period of time.
The seismic signal generated by a seismic vibrator is a controlled wavetrain (i.e., a sweep) which is applied to the surface of the earth or in the body of water or in a borehole. In seismic surveying on land using a vibrator, to impart energy into the ground in a swept frequency signal, the energy is typically imparted by using a hydraulic drive system to vibrate a large weight (the reaction mass) up and down. The reaction mass is coupled to a baseplate, in contact with the earth and through which the vibrations are transmitted to the earth. The baseplate also supports a large fixed weight, known as the hold-down weight. Typically, a sweep is a sinusoidal vibration of continuously varying frequency, increasing or decreasing monotonically within a given frequency range, which is applied during a sweep period lasting from 2 to 20 seconds or even more. The frequency may vary linearly or nonlinearly with time. Also, the frequency may begin low and increase with time (upsweep), or it may begin high and gradually decrease (downsweep).
The seismic data recorded during Vibroseis prospecting (hereinafter referred to as “vibrator data”) are composite signals, each consisting of many long, reflected wavetrains superimposed upon one another. Since these composite signals are typically many times longer than the interval between reflections, it is not possible to distinguish individual reflections from the recorded signal. However, when the seismic vibrator data is cross-correlated with the sweep signal (also known as the “reference signal”), the resulting correlated data approximates the data that would have been recorded if the source had been an impulsive energy source.
The amount of energy injected into the earth during a conventional vibrator sweep is governed by the size of the vibrator and the duration of the sweep.
There are several of constraints on the amplitude of the vibrations. The most basic of these is that the hold-down weight must exceed the maximum upward force, so that the vibrator never loses contact with the ground. However, there are other constraints on low frequency output. Since, as already mentioned, the ground force is generated by vibrating a large weight, and the force generated by the weight is equal to its mass times its acceleration, at low frequencies for the same generated ground force the peak velocities and displacements are higher than at high frequencies. Typically, the lowest frequency that can be produced by a vibrator at a fixed force level is determined by the maximum stroke length possible for the vibratory weight, and the amount of time that the vibrator can spend at low frequencies is determined by the amount of hydraulic fluid stored in accumulators at the start of the sweep time and the maximum flow capacity of the hydraulic system.
Vibrators for use in marine seismic surveying typically comprise a bell-shaped housing having a large and heavy diaphragm, equivalent to the aforementioned baseplate, in its open end. The vibrator is lowered into the water from a marine survey vessel, and the diaphragm is vibrated by a hydraulic drive system similar to that used in a land vibrator. Alternative marine vibrator designs comprise two solid curved or hemispherical shells, joined together by an elastic membrane. The hydraulic drive moves the two shells relative to one another in a similar manner to the movement of the reaction mass in a land vibrator. Marine vibrators are therefore subject to operational constraints analogous to those of land vibrators.
Another problem with conventional Vibroseis prospecting results from the fact that vibrators generate harmonic distortion as a result of nonlinear effects in the vibrator hydraulics and the ground's nonlinear reaction to the force exerted by the vibrator baseplate, with the second and third harmonics accounting for most of the distortion. These harmonics are present in the recorded data and lead to trains of correlated noise, known as harmonic ghosts, in the correlated data. These harmonic ghosts are particularly troublesome in the case of downsweeps where they occur after the main correlation peak (i.e., positive lag times) and, therefore, can interfere with later, hence weaker, reflections. In the case of upsweeps, harmonic ghosts are somewhat less troublesome because they precede the main correlation peak (i.e., negative lag times). Nevertheless, harmonic ghosts can cause difficulties in processing and interpreting data from upsweeps as well as from downsweeps.
U.S. Pat. No. 5,410,517 issued to Andersen discloses a method for cascading or linking vibrator sweeps together to form a cascaded sweep sequence and optionally eliminating the listen period between successive sweeps. The initial phase angle of each individual sweep segment within a sweep sequence is progressively rotated by a constant phase increment of about 360/N degrees, where N is the number of sweep segments within the sweep sequence. Either the correlation reference sequence or the vibrator sweep sequence, but not both, contains an additional sweep segment positioned and phased so as to substantially suppress harmonic ghosts during correlation. When the additional sweep segment is included at the end of the vibrator sweep sequence, it increases the total acquisition time. If the correlation reference sequence includes the additional sweep segment, it complicates the processing in that the additional sweep segment has to be input at negative time giving a nonstandard correlation operator.
In the United Kingdom published patent application GB-A-2387226 there is disclosed a method of seismic acquisition using multiple vibrators using the so-called “slip-sweep” method. The method consists of a vibrator (or a vibrator group) sweeping without waiting for the previous vibrator's sweep to terminate. Correlation, which acts as a time-frequency filter, then extracts the individual records. A significant reduction in overall acquisition time is obtained. This is more efficient than the cascaded sweep since there is no need to wait for the end of a sweep before starting the next sweep. The reduction in overall acquisition time comes at the cost of increased harmonic distortion since the harmonics from the second sweep will correlate with the primary signals of the first sweep.
U.S. Pat. No. 6,418,079 issued to Fleure discloses a method for segmenting the spectral distribution of overlapped vibratory signals, thereby improving the efficiency of data acquisition while providing reduced harmonic distortion in the time zones of interest. Two identical sweep segments are used. Each sweep segment includes an earlier low frequency sweep and a later high frequency sweep, the individual sweeps having substantially no overlap in frequency except for tapering. The high frequency sweep in each pair starts before the end of the low frequency sweep with an overlap in time that is selected to avoid harmonics from the low frequency sweep. Correlation of the recorded signal separately with the low frequency sweep and the high frequency sweep gives data sets in which individual portions of the desired data are recoverable with the harmonic distortion largely separated from the desired data.
Another prior art way of seeking to overcome the problems resulting from the various constraints on land or marine vibrator operation is disclosed in U.S. Pat. No. 6,181,646. The vibrator source of the system (hereinafter referred to as the prior art system) described in that patent is driven so as to provide a composite sweep, in which a high frequency sweep and a low frequency sweep are carried out concurrently over the same time interval, i.e., both sweeps start at the same time and finish at the same time.
While the prior art system has several advantages, it also suffers from a number of drawbacks.
Firstly, starting both high and low frequency sweeps at the same time limits the force that can be generated at the bottom of the high frequency band. At this point in the sweep, the high-frequency sweep is limited by the hydraulic and stroke limitations of the vibrator, but to add a low-frequency sweep at the same time reduces the available resource for the high-frequency sweep.
Secondly, hydraulic vibrators inevitably generate energy not just at the desired frequency, but also at harmonics of that frequency. Harmonics of the low frequency sweep will lie in the same frequency band as the fundamental of the high-frequency sweep. If those harmonics are emitted between the time when the same frequency is emitted by the high-frequency sweep and the end time of the seismogram derived from the sweeps, then the harmonics will be interpreted as seismic signal and contaminate the seismogram.
Thirdly, in order to have complete spectral coverage, it is desirable that there should be some overlap between the high and low frequency sweeps. As a result, correlating the reflected seismic signals with the summed sweeps, i.e. with the drive to the vibrator, may lead to artifacts appearing in the seismogram at the overlap frequencies and reduces the opportunity for spectral balancing.
There is a need for an invention that acquires data with increased efficiency by using overlapping sweeps while providing some measure of protection against harmonics.